Sea-Level Rise Accelerating on U.S. Atlantic Coast

Rates of sea-level rise are increasing 3 to 4 times faster along parts of the U.S. Atlantic coast than globally, according to a U.S. Geological Survey (USGS) report published in Nature Climate Change in June 2012 by Asbury (Abby) Sallenger, Kara Doran, and Peter Howd.

Since about 1990, sea-level rise in the 1,000-km (600 mile) stretch of coastal zone from Cape Hatteras, North Carolina, to north of Boston, Massachusetts—coined a “hotspot” by the authors—has increased 2 to 3.7 millimeters per year; the global increase over the same period was 0.6 to 1.0 millimeter per year. Data and analyses included in the report indicate that if global temperatures continue to rise, rates of sea-level rise in this area are expected to continue increasing.

The report shows that the sea-level-rise hotspot is consistent with the slowing of Atlantic Ocean circulation. Models show that this change in circulation may be tied to changes in water temperature, salinity, and density in the subpolar North Atlantic.

“Many people mistakenly think that the rate of sea-level rise is the same everywhere as glaciers and ice caps melt, increasing the volume of ocean water, but other effects can be as large or larger than the so-called ‘eustatic’ rise,” said USGS Director Marcia McNutt. “As demonstrated in this study, regional oceanographic contributions must be taken into account in planning for what happens to coastal property.”

Above: Differences in rates of sea-level rise from tide-gauge records across North America over a 60-year period (1950–2009). Circles are color coded to reflect computed differences; no color fill indicates differences in rates of sea-level rise that are not statistically different from zero. Cool colors indicate decreasing rates of sea-level rise over the 60-year period; warm colors indicate increasing rates of sea-level rise over the 60-year period. Note “hotspot” between Cape Hatteras and Cape Cod. After figure 2 in “Hotspot of Accelerated Sea-Level Rise on the Atlantic Coast of North America.” [larger version]

Although global sea level has been projected to rise about 1 meter (3 feet) or more by the end of the 21st century, it will not climb at the same rate everywhere. Differences in land movements, strength of ocean currents, water temperatures, and salinity can cause regional and local highs and lows in sea level.

“Cities in the hotspot, such as Norfolk, New York, and Boston, already experience damaging floods during relatively low intensity storms,” said Abby Sallenger, USGS oceanographer and project lead. “Ongoing accelerated sea-level rise in the hotspot will make coastal cities and surrounding areas increasingly vulnerable to flooding by adding to the height that storm surge and breaking waves reach on the coast.”

To determine accelerations of sea-level rise, USGS scientists analyzed tide-gauge data throughout much of North America in a way that removed long-term (linear) trends associated with vertical land movements. This analysis allowed them to focus on recent changes in rates of sea-level rise, caused, for example, by changes in ocean circulation.